1. Field of the Invention
The present invention relates generally to polyolefin production and, more specifically, to increasing recovery of unreacted olefin monomer discharged from a polyolefin reactor.
2. Description of the Related Art
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and/or the chemical requirements, such thermal stability, molecular weight, and chemical reactivity, typically determine what polyolefin or type of polyolefin is suitable.
One benefit of polyolefin construction, as may be deduced from the list of uses above, is that it is generally non-reactive with goods or products with which it is in contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. Typically, these processes are performed at or near at petrochemical facilities, which have ready access to the short-chain olefin molecules (monomers and comonomers) such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefin polymers. These monomers and comonomers may be polymerized in a liquid-phase polymerization reactor and/or gas-phase polymerization reactor to form a product comprising polymer (polyolefin) solid particulates, typically called fluff or granules. The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), copolymer content, comonomer content, modulus, and crystallinity. The reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, and so forth, may be selected to achieve the desired fluff properties.
In addition to the one or more olefin monomers, a catalyst for facilitating the polymerization of the monomers may be added to the reactor. For example, the catalyst may be a particle added via a reactor feed stream and once added, suspended in the fluid medium within the reactor. An example of such a catalyst is a chromium oxide containing hexavalent chromium on a silica support. Further, a diluent may be introduced into the reactor. The diluent may be an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and n-hexane that is liquid at reaction conditions. However, some polymerization processes may not employ a separate diluent, such as in the case of selected examples of polypropylene production where the propylene monomer itself acts as the diluent.
The discharge of the reactor typically includes the polymer fluff as well as non-polymer components, such as unreacted olefin monomer (and comonomer), diluent, and so forth. In the case of polyethylene production in liquid phase reactors, such as loop slurry reactors, the non-polymer components typically comprise primarily diluent, such as isobutane, having a small amount of unreacted ethylene (e.g., 5 wt. %). For polypropylene production, the non-polymer components typically comprise primarily unreacted propylene monomer. These discharge streams are generally processed, such as by a diluent/monomer recovery system, to separate the non-polymer components from the polymer fluff. The recovered diluent, unreacted monomer, and other non-polymer components from the recovery system may be treated, such as by a fractionation system, and ultimately returned as purified or treated feed to the reactor. In some cases, the components may be flared or returned to the supplier, such as to an olefin manufacturing plant or petroleum refinery. As for the recovered polymer (solids), the polymer may be treated to deactivate residual catalyst, remove entrained hydrocarbons, dry the polymer, and pelletize the polymer in an extruder, and so forth, before the polymer is sent to customer.
The competitive business of polyolefin production continuously drives manufacturers to improve their processes to lower production costs. In an industry where billions of pounds of polyolefin product are produced per year, small incremental improvements, for example, in catalyst activity, monomer yield, and diluent recovery, can generate significant cost savings in the manufacture of polyolefins. Fortunately, technological advances over the years in raw materials, catalyst productivity, energy efficiency, equipment design and operation, and the like, have provided great strides in reducing the capital, operating, and fixed costs of polyolefin manufacturing systems. For example, catalyst research has produced commercial catalysts with activity values that are orders of magnitudes higher than those of two to three decades ago, resulting in a striking reduction in the amount of catalyst consumed per pound of polymer, and also reducing the downstream processing (and equipment) used to deactivate and/or remove residual catalyst in the polymer product. Further, advances in equipment design and operation have also greatly increased diluent recovery to the point where very little fresh diluent make-up is utilized.
Technological advances have also improved monomer yield, which is a measure of the conversion of monomer, such as ethylene or propylene, to a polymer or polyolefin, such as polyethylene or polypropylene. Ideally, one pound of monomer produces one pound of polyolefin, but typical monomer-yield values (expressed as the ratio of pounds of polymer produce per pound of monomer consumed) in the industry generally hover around 95%. Further increases in monomer yield provide one of the greatest opportunities to reduce the cost to manufacture polyolefins. Indeed, olefin monomer is typically the largest cost in producing polyolefins, with small incremental improvements in monomer yield resulting in considerable cost savings. Such desired improvements in monomer yield, however, are a significant challenge in the polyolefin industry.
The polyolefin industry's struggles to increase monomer yield further are due, in part, to the difficulty of separating small amounts of monomer entrained in light component streams and vent/purge streams throughout the polyolefin process, such as in the diluent/monomer recovery and fractionation sections of the polyolefin plant. Separation of entrained monomer from these light streams is typically not feasible for a variety of reasons. As appreciated by those of ordinary skill in the art, theoretical limitations, such as azeotropes, pinch points, etc., in the separation equilibrium, for example, may preclude separation. Further, where theoretically possible, separation of the entrained monomer from the vent streams would generally require capital-intensive investments not having a justified economic return, even with the significant savings in recovered ethylene. For example, the separation and recovery of the entrained monomer may require installation of fractionation columns having design dimensions (e.g., diameter, height, number of stages or trays, reflux flow rates, etc.) that are surprisingly large. Thus, these vent streams having small concentrations of monomer, such as ethylene and propylene, are typically combusted in a flare or recycled to the olefin supplier.
A problem with incineration of the vent streams at the flare is that the entrained monomer is lost. A problem with recycling of the monomer to the olefin supplier is that the supplier often cannot effectively process the return stream because of the presence of undesirable components, such as inert components. In fact, the polyolefin producer generally receives a reduced credit for the returned monomer due to the difficult processing requirements experienced by the supplier. Even more unfortunate is that the supplier themselves may have to flare the return stream due to their inability to process the stream, and thus the polyolefin producer receives no credit for the entrained monomer in the recycle stream.